This invention relates, in general, to a method of bonding leads to a semiconductor chip, including, but not limited to a method of controlling the deformation of the bond.
Tape Automated Bonding (TAB) is usually done by gang bonding of a plurality of leads attached to a tape to bonding surfaces or bumps on a semiconductor chip. This is done utilizing a means for applying heat and a constant force to the plurality of leads of the tape and the bumps on the chip. The means for applying heat and force is a bonding tool, commonly referred to as a thermode, positioned on a TAB bonder. During bonding, the TAB leads and the bumps are deformed by the constant force applied by the bonding tool. The plurality of TAB leads are typically tin plated and the bump is typically gold. A gold-tin eutectic bond is formed between the lead and the bump.
In the past, some TAB bonded devices have experienced some problems in reliability due to factors that vary in the bonding process. Factors that can vary within the bonding process include, among others, bond area and hardness of the materials comprising the lead and the bump.
The bond area varies because the incoming TAB lead size can vary as much as +/-10% and the bump size can vary as much as +/-5%. This means that the bond area can vary as much as +/-15%. The variation in the bond area generates a +/-15% variation in stress on the TAB leads bumps and semiconductor chip, because the force applied to the leads by the bonding tool during bonding is constant. This variation in stress causes a large variation in the amount of deformation of the TAB leads and bumps due to the relatively flat slope of the gold modules curve at the bonding temperature, which is typically greater than 300.degree. C. The typical variation in the hardness of the gold bumps is +/-10%. This variation in the hardness of the gold bumps also generates a variation in the stress on the TAB leads and the bumps.
Because of both of these variations, it is necessary for the bonder operators to adjust the bonder force or bonding temperature depending on the variation of the bond area and the hardness of the materials. If the proper adjustments are not made, the TAB leads can be under- or over-bonded, thereby causing rejects. An under-bonded lead is one in which the lead is not deformed enough so that the lead and the bump do not adhere properly. An over bonded lead is one in which the lead is deformed too much, which can result in reduced bond strength and shorting of the lead to other parts of the semiconductor chip.